The reason, the passenger explained, was the B-110 caters to the Hasidic Jewish community running between Borough Park and Williamsburg and tradition calls for men and women to be separated.

The bus driver then asked Brown to move as well. When she resisted he seemed confused but returned to his seat.

But, one by one, passengers continued to approach Brown, telling her she could not sit in the front and needed to move to the back.

The B110 is a public bus and looks similar to other city buses. Riders pay an amount equal to Metropolitan Transportation Authority fares of $2.50, cash only, no MetroCard.

But it is what’s known as a franchise route, meaning a private company pays the city for the right to provide the public service.

After getting several complaints from passengers who had experiences similar to Brown’s — the Department of Transportation now says it is investigating the bus company — Private Transportation Corp. — for discrimination.

In a letter to the bus company’s owner the DOT says its been made aware of “serious allegations” and advises the owner that “(the) practice of requiring women to ride in the back, or allow passengers to harass women who choose to ride in any part of the bus, is not permitted on franchise buses…”

The letter goes on to say any such practice is a direct violation of their agreement.

Brown visited the bus company for its side of the story but was told no one would be available to answer her questions until next week.

A stern Mayor Michael Bloomberg was blunt about the policy.

“That is obviously not permitted,” he said, adding it may be permissible if it were a private bus.

The bus company pays the city $22,000 to operate the franchise service. The DOT says it has reached out to the company and expects an explanation, as well as a plan to prevent future cases of discrimination, by Oct. 26.